From the top of my head I can think of several different ways to do that with a single MC13 instance even without resorting to other MC/MJ instances or external tagger programs.
Marko's solution would work, but removing and rewriting all tags can cause heavy file rewriting if the possibly existing empty padding area is completely used.
A couple of alternative solutions:
- Enable tag writing in the Rating field's settings, use the "Move / Copy Fields" tool and move the Rating values to some other field (set the target field to not write tags), disable tag writing in the Rating field's settings, move the values back to the Rating field.
- Disable Auto-Import, create a cloned copy of the library (use the "Clone From" option in Library Manager), load the cloned library, enable tagging in the Rating field's settings and unrate the files. After that you can load the normal library and set your normal options. You can delete the cloned library.